This invention relates to a method for positioning two objects relative one another, and more particularly for positioning and maintaining a disc head over a desired track on a disc recording surface.
Disc drives are frequently used as secondary storage for computing systems. A typical disc drive includes a rotating spindle having one or more recording surfaces fixedly mounted thereon for rotation therewith. Data is written onto the disc in concentric circles denominated tracks by a head positioned closely adjacent the recording surface. This recorded information may subsequently be read from the disc using either this same head or a second head specially adapted for reading.
The recording head is typically fixedly mounted on an arm capable of moving radially with respect to the rotating surface. The individual tracks on the recording surface are accessed by positioning the head over the desired track by moving the disc arm radially. The repositioning of the head from one track to another is referred to as a seek. Frequently, these seeks limit the speed of the overall computing system because they are mechanical operations whereas the remainder of the computing system typically operates at electronic speeds. Therefore, it is extremely desirable to have the head move rapidly and accurately to the target track on each seek. However, there is a trade-off between the velocity of head movement and the accuracy of the landing position over the desired track. If the head is moved at a relatively high velocity, it acquires a certain momentum which if not properly arrested causes the head to overshoot or pass over the desired track rather than stopping on this desired track. When this happens, it is necessary to move the head in the reverse direction to correct this overshooting.
Consequently, methods for moving the disc arm and head vary greatly in their trade-off between speed and overshooting. Some algorithms favor extremely high head velocities claiming that the time required to fine tune the head over the desired track after the major portion of the movement is offset by the time saved moving the head at the high velocity. Other algorithms prefer to move the head more slowly so that no overshoot results when the desired track is reached. Proponents of these algorithms maintain that the time saved in settling the head over the track is better spent moving the head more slowly.
Embedded servo technology has greatly increased the track-packing densities previously available without such technology. With embedded servos, servoing information is placed on the disc enabling the head to very precisely follow the centerline of each data track. Typically, embedded servo bursts are sampled and a positional error signal is generated which is fed back to the disc-arm-moving mechanism to precisely position the head over the centerline of the track.
Typically, prior artisans have treated the seek function and the follow function as two separate functions. That is to say that one module of the disc drive unit is responsible for seeking a desired track and a second module is responsible for maintaining the head in proper alignment over that track. The inclusion of these two modules renders these drives relatively complex and consequently expensive.